Falling is a significant problem in the care of the elderly that can lead to morbidity and mortality. From a physical perspective, falls cause injuries, while from the mental perspective, falls cause fear-of-falling, which in turn leads to social isolation and depression.
Fall detection devices and systems are available that can provide an automated and reliable means for detecting when a user has fallen. If a fall is detected, the device or system issues an alarm which summons help to the user. This assures the user that adequate measures will be taken in the event that a fall occurs.
Commonly, fall detectors are based on an accelerometer (usually a 3D accelerometer that measures acceleration in three dimensions) that is part of a device to be attached to the user's body. The signals from the accelerometer are processed to determine whether a fall has taken place.
The reliability of fall detection can be improved by making use of further sensors which can be used to detect various different features that are characteristic of a fall. Important features include the impact of the user with the ground during the fall, an orientation change as the user falls, and a reduction in the height of the sensor unit above the ground. In EP 1642248, the use of an air pressure sensor is proposed to detect a change in the relative height measured by the device.
Currently available air pressure sensors provide a resolution in the relative altitude of the order of 10 cm. However, the nature of these pressure sensors means that their measurements are sensitive to gravity, and hence to the orientation of the sensor unit. This can be addressed by compensating the pressure sensor measurements for the orientation of the sensor unit, as described in WO 2009/101566. In addition, air pressure sensors clearly also respond to barometric fluctuations in the environment, and therefore the fall detector needs to verify whether the height change suggested by an increase in air pressure measurements is due (or can be due) to the motion of the sensor unit and the user. A further problem with air pressure sensors is that they increase the complexity of the mechanical construction of the device that houses the sensors. In particular, the device is required to have a fast-responding channel between the air pressure sensor inside the device and the environmental air outside, with this channel also being shielded against moisture, light, and other pollution.
Another approach to determining a measurement for the change in height is to use the accelerometer signal. By integrating the vertical acceleration signal, a measure for vertical velocity can be obtained, and by integrating the vertical velocity signal, a measure for position/height can be obtained. The integration typically requires knowledge of the initial vertical velocity and initial position/height.
Since in fall detection one aim is to detect a change in height, i.e. a difference between two positions in time, the integration can in fact be performed without knowledge of the value of the initial position, since it cancels in the difference equation.
In addition, in fall detection, the initial vertical velocity is zero, provided the “initial” time moment is correctly chosen. In common daily situations, this can be any point before the onset of the fall. It will be noted, however, that any deviation from zero of the true, physical vertical velocity integrates over the chosen range to an error in the position/height estimation.
However, another problem in using double integration of an accelerometer signal concerns the proper separation of the acceleration due to gravity from the component of acceleration due to the motion of the user. To achieve a precision in height measurement of 10 cm over 1 second, the residual gravity component in the acceleration signal should stay within 0.2 ms−2. Given that gravity is approximately 10 ms−2, gravity needs to be separated with an accuracy of a few percent.
Since the orientation of the sensor unit is likely to be changing as the user falls, the direction of the vertical in terms of the coordinate system of the sensor unit will be changing as well. Here, the same problem arises. An error in the orientation estimation causes an error in the computation of the non-gravitational vertical component of acceleration. For the same reason, an error in the orientation also implies an error in the calculation of the corresponding gravity component. These errors manifest themselves in the vertical velocity estimate and therefore also in the estimated height (or change in height) of the device.
Furthermore, if the acceleration sensor is not properly calibrated, or has lost calibration over time, the sensed gravity will also change with the orientation of the sensor unit.
Therefore, there is a need for an improved method and apparatus for measuring or estimating the velocity of a device in a horizontal or vertical direction based on measurements of the acceleration experienced by the device that overcomes the above problems.